Description of the System
In the MDGI Joist System, conventional open web joists are slightly modified in fabrication to allow for composite action to take place with a poured in place concrete slab. In addition, the joist end bearing is designed to be encased in the slab and act as a shear connector. This allows composite action to take place in the structural steel support with considerable weight saving beams as well. Other joist systems do not have this advantage the floor system becomes composite in both directions producing a stiffer yet lighter floor.
The composite joists are conventionally designed with top and bottom chord angles and round bar webs. They look like standard joists except that the webs project above the top chord angles. The system 1-inch deep corrugated steel sheets are in long lengths and are pre-punched with slotted holes along their centerline and spaced to match the project joist spacing. The sheets span continuously over multiple joists. The joists can be at any spacing up to 4’-0” on centers. To lock down the sheets and help to create the necessary bond between joists and concrete, steel wedges are inserted within the curved web projections. They can be hammered in place or welded or power fastened through the wedge and the decking to the top of the top chord angles. Just a few wedges will hold a sheet in place until it is permanently secured to the joists. This also prevents blow offs during erection which is an advantage in multistory building.
The composite joists are conventionally designed with top and bottom chord angles and round bar webs. They look like standard joists except that the webs project above the top chord angles. The system 1-inch deep corrugated steel sheets are in long lengths and are pre-punched with slotted holes along their centerline and spaced to match the project joist spacing. The sheets span continuously over multiple joists. The joists can be at any spacing up to 4’-0” on centers. To lock down the sheets and help to create the necessary bond between joists and concrete, steel wedges are inserted within the curved web projections. They can be hammered in place or welded or power fastened through the wedge and the decking to the top of the top chord angles. Just a few wedges will hold a sheet in place until it is permanently secured to the joists. This also prevents blow offs during erection which is an advantage in multistory building.
The joist bearing ends are designed to receive preformed fitted pans which are dropped down to bear on the joist top chord angles and the top of the supporting steel girders. The pans fit between adjacent joists and when the concrete slab is poured a thickened haunch is formed over the supporting girders. When additional composite capacity is required, conventional shear studs can be added within the haunch.
Where composite joists are supported on cold formed steel stud bearing walls, the concrete haunch helps distribute the joist bearing loads uniformly along the top of the wall. It also acts as a fire and sound barrier from bay to bay.
The slab is reinforced with conventional wire mesh spanning continuously joist to joist. Because the joist webs become anchored in the floor slab, little or no bridging is required.
Where composite joists are supported on cold formed steel stud bearing walls, the concrete haunch helps distribute the joist bearing loads uniformly along the top of the wall. It also acts as a fire and sound barrier from bay to bay.
The slab is reinforced with conventional wire mesh spanning continuously joist to joist. Because the joist webs become anchored in the floor slab, little or no bridging is required.
Other Joist Systems
Our MDGI Joist System is unique. There are two basic composite joist approaches with other systems. The first uses a corrugated stell deck layed at right angles to and continuous over conventional joists. Composite action with the concrete slab is accomplished with shear studs which are welded through the decking and onto the top flanges of the joist top chord angles. The welding operation is difficult to accomplish since the top chords are hidden from view and are narrow and relatively thin, which also limits the design of the studs that can be used. There is a similar problems with composite joists using screw stud devices. The MDGI Joist System solves these problems. The projecting web shear configuration is fabricated as part of the joist.
The second popular design has the joist top chords partially embedded in the concrete slab for composite action. This requires plywood or corrugated decking in short lengths which span joist to joist. Corrugated decking remains in place. Plywood requires removal after the pour. Continuous cracking of the slab over the joist can be a problem because the top chord embedment reduces the slab thickness over the joist. The MDGI Joist System carries the full slab thickness over the joists, avoiding longitudinal cracking.
The second popular design has the joist top chords partially embedded in the concrete slab for composite action. This requires plywood or corrugated decking in short lengths which span joist to joist. Corrugated decking remains in place. Plywood requires removal after the pour. Continuous cracking of the slab over the joist can be a problem because the top chord embedment reduces the slab thickness over the joist. The MDGI Joist System carries the full slab thickness over the joists, avoiding longitudinal cracking.
Parking Decks
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Our design has been successful in construction of parking decks beneath buildings and free standing structures. The long term advantage is the ease of inspection to guard against deterioration and the ease of minor repairs. The entire structural steel framing is accessible for inspection, painting, and minor repairs if necessary. Where the joists are galvanized, they have a very long life before requiring attention. The one-inch galvanized decking is only used as a form, but also has a long life. Generally the concrete slab, which is usually 4” thick overall with wire mesh, experiences the most attack due to exposure, temperature, salts, etc. Deteriorated concrete can be removed and replaced without disturbing the steel structure.
The system is particularly economical for small or oddly-shaped parking structures. |
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Call us today for a quote!
We can quote our composite system, in most areas, which includes composite joists and prepunched decking (with accessories). In selected areas, we can also include the basic steel frame supporting the composite system. We can even include shop details and design assistance where necessary!
